DE102008008238B4 - Method for the charging strategy of a hybrid drive and implementing control unit - Google Patents

Abstract

Method for the charging strategy of a hybrid drive (1) by means of an internal combustion engine (2), an electric machine (3), a battery and at least one control unit,
- various charging and discharging functions for the battery are stored in the control unit,
- depending on various input variables, the control unit selects one of the various charging or discharging functions, which is then adjusted by a load point shift on the internal combustion engine (2) and the electric machine (3),
- wherein the input variables are at least a current difference between an actual state of charge and a target state of charge, a distance from the actual state of charge to state of charge min/max limits and current operating points of the internal combustion engine (2) and the electric machine (3) as well as their efficiency maps,
- in addition to the charging and discharging functions, a 0-charging function and a pulse charging function are also stored in the control unit,
- wherein the target charge state is adjusted during operation of the hybrid drive (1),
- whereby output variables of the control unit are subjected to a correction, whereby further system limits and/or special operating conditions are taken into account and whereby correction information is fed back as input variables.

Description

The invention relates to a method for the charging strategy of a hybrid drive and a control unit suitable for carrying out the method.

Hybrid vehicles combine two drive units that provide power for the vehicle's propulsion in different ways. The properties of an internal combustion engine and (at least) one electric motor complement each other particularly well, which is why hybrid vehicles today are predominantly equipped with this combination. The electric motor can be connected to the engine crankshaft in various ways. For example, it can be connected via a clutch or directly to the crankshaft of the internal combustion engine, or it can be coupled via a belt drive or a transmission. The electric motor is controlled by a control unit (inverter, power electronics).

Deep-cycle batteries using NiMH or Li-ion technology are often used in hybrid vehicles as electrical energy storage devices. The service life of a battery is limited, among other things, by the energy transferred (i.e., stored and released) as well as the power or energy swing during the charging and discharging processes. Extensive cycling thus leads to a reduced service life.

To achieve a sufficient battery service life, it is therefore necessary to limit fluctuations in the state of charge (SOC) and, if possible, to keep the battery's SOC (state of charge) within a target range defined by lower and upper limits (SOCmin and SOCmax). This means that both very high and very low states of charge must be permanently avoided, as this significantly reduces the battery's effectively usable SOC range. Typically, the aim is to maintain the state of charge of hybrid batteries within a window of +-30%, preferably +-20%, and particularly preferably +-10%, of the target state. The preferred SOC ranges can also be distributed asymmetrically around the target state of charge.

The achievable reduction in fuel consumption in hybrid vehicles results crucially from a suitable control strategy that avoids operation in areas of low combustion engine efficiency as far as possible or advantageously combines the efficiency characteristics of the combustion engine and the electric motor.

For example, purely electric propulsion can be used in areas with low load requirements, where a combustion engine typically has low efficiency. This type of operation is particularly effective when the combustion engine and electric motor can be mechanically decoupled from each other, for example, via an additional clutch. Additional fuel consumption potential arises, for example, from a start-stop function and the possibility of braking energy recovery.

Ideally, the majority of the energy required to propel the vehicle and supply the on-board electrical system is generated from upstream recuperation processes and temporarily stored in the energy storage system. To achieve this, the vehicle's deceleration phases are utilized by generating as much of the required braking power as possible through the generator operation of the electric motor. In real driving cycles, however, achieving the target charge level usually requires the generation of additional electrical energy via the combustion engine and generator operation of the electric motor.

In order to be able to store as much electrical energy as possible through recuperation in favorable driving situations (frequent deceleration or braking, downhill driving), a sufficient distance between the current charge level and the upper permissible value is required.

Against this background, it makes sense to keep the target state of charge low in order to be able to store the recuperation energy as completely as possible and thus to carry out the generator charging process as rarely as possible (or with low power) via the load point shift of the combustion engine.

In addition, the electric motor in the drivetrain is often used to improve the performance of the hybrid vehicle. The torque output of the combustion engine and electric motor can also occur in parallel, for example, to increase the maximum torque of the entire drive unit (boost mode).

In order to demonstrate reproducible driving behavior, it is important to always display the boost function with the same performance under comparable boundary conditions.

In particular, high electrical power consumption leads to a rapid approach to the lower permissible SOC value of the energy storage device and subsequently requires corresponding charging phases in order to To avoid functional limitations due to an inadmissibly low charge level.

For driving operations with frequent boosting processes, it is therefore advantageous to set the battery's target charge level as high as possible in order to create an appropriate energy reserve for the discharging processes.

In the hybrid control system of implemented hybrid vehicles, a target value or target range for the battery's state of charge is calculated, taking into account the fundamental relationships outlined above as well as additional influencing factors such as thermal limits, current or voltage limits, aging effects, etc. The task of a charging function is to appropriately adjust the state of charge of the energy storage device, depending on the driving strategy, e.g., by increasing the load point of the combustion engine and operating the electric motor in generator mode to charge the energy storage device (actual SOC less than target SOC), according to the calculated target value or target range.

Furthermore, in driving situations with a high proportion of recuperation and infrequent or low-performance discharge phases, it may also be necessary to use the charging function to specifically reduce the charge level of the energy storage device (actual SOC greater than target SOC). This can be achieved, for example, by providing electric motor drive assistance with the combustion engine running (and the disconnect clutch engaged) accordingly. 1 to substitute part of the combustion engine drive power.

In real driving operation of the hybrid vehicle, there is the additional difficulty that during the charging phases, in order to demonstrate a largely reproducible driving behavior, other hybrid functions (boost, recuperation, electric driving, start-stop) are also active and thus continuously influence the charge state of the energy storage device.

At the same time, the charging strategy must also make it possible to achieve an optimum in terms of consumption or driving performance or an optimal compromise between the two.

From the WO 2006 / 053 624 A1 A method for controlling the operation of a motor vehicle with a hybrid drive unit is known, which comprises an internal combustion engine and at least one electrical machine which can be operated either as a motor or as a generator, wherein the electrical machine charges an energy storage device in generator mode and/or supplies an electrical system of the motor vehicle, wherein, depending on compliance with predetermined boundary conditions, the operation of the electrical machine in generator mode takes place in alternating intervals, wherein in a first interval the electrical machine is operated with a first, high electrical power output which is higher than a current power consumption of the electrical system and is switched off in a second interval.

The DE 10 2007 028 700 A1 discloses a method and a device for controlling the operation of a hybrid vehicle, comprising an internal combustion engine, at least one electric machine, at least one energy storage device and at least one control unit, wherein the generator charging power of the electric machine for charging the energy storage device is reduced as a function of at least one vehicle state parameter and a distance between a desired state of charge (desired SOC) and an actual state of charge (actual SOC).

The JP H10 - 150 701 A1 describes a control unit that controls the conversion of power output from an internal combustion engine by means of a generator into electrical energy for charging a battery. The power is output to a drive shaft of the internal combustion engine according to the measurement of the operation of an accelerator pedal using the electrical energy stored in the battery. A CPU of the control unit calculates a difference between an input state SOC and a target state SOC. It then compares the difference with a first threshold and a second threshold. If the difference is smaller than the first threshold, it is determined that the battery needs charging and charging is started. If the difference is greater than the second threshold, the CPU determines that the battery does not need charging and stops charging.

The DE 698 15 471 T2 describes a method for operating an electric hybrid vehicle that draws at least part of its drive power from electric batteries. At least in one operating mode of the electric vehicle, energy is supplied from a traction battery to a traction motor. The method includes returning at least part of the energy provided by regenerative braking to the batteries, with the traction motor operating as an electric generator, and charging the batteries from an auxiliary electrical source separate from the traction motor during intervals in which no dynamic braking is performed.

The US 5 786 640 A discloses a hybrid vehicle with a sensor for detecting the state of charge (SOC) of a battery, wherein the power of a generator or an internal combustion engine is controlled based on the SOC and its change.

The invention is based on the technical problem of creating an improved method for the charging strategy of a hybrid drive and a control unit carrying out the method.

The solution to the technical problem results from the objects having the features of claims 1 and 4. Further advantageous embodiments of the invention result from the subclaims.

For this purpose, the control unit is designed to carry out a method for the charging strategy of a hybrid drive in such a way that various charging and discharging functions are stored in the control unit, wherein the control unit selects one of the various charging or discharging functions depending on various input variables, which is then set by a load point shift on the internal combustion engine and the electric machine, wherein the input variables include at least the current difference between the actual SOC and the target SOC, a distance between the actual SOC and the SOC min/max limits, the current operating points of the internal combustion engine (e.g. speed and/or torque) and the electric machine (e.g. speed and/or torque and/or current and/or voltage) and their efficiency maps.

The ability to switch between different charging and discharging functions during operation allows for better situational responses to varying fuel consumption requirements and reproducible hybrid drive behavior. Preferably, the input variables are assigned importance or weighting factors, thus enabling the selection to be made using a type of fuzzy logic.

The basic calculation of the battery charge or, in general, of the electrical energy storage of the various charging and discharging functions by load point shifting can preferably be carried out via a characteristic map which receives as input variables, among other things, the speed of the internal combustion engine and the electric motor as well as the internal combustion engine torque before the load point shift.

In a preferred embodiment, further battery parameters are taken into account as additional input variables, such as in particular the temperature and/or the state of health of the battery (SOH), which is also referred to as an indicator of the degree of battery wear, gear information, vehicle speed, the state of any separating clutch, and/or other operating parameters of the internal combustion engine and/or transmission. The list is to be understood in such a way that at least one of the aforementioned parameters is additionally taken into account in the sense of an and/or connection. This allows the charging strategy to be optimally adapted even better to the overall condition. The efficiency maps are preferably stored permanently in the control unit.

According to the invention, in addition to the charging and discharging functions, a 0-charging function and a pulse charging function are also stored in order to react appropriately to conditions in which only small deviations occur between the actual SOC and the target SOC, whereby with regard to the pulse charging function, explicit reference is made to the WO 2006 / 053 624 A1 The zero-charge function offers the advantage that the electrical energy storage device is not stressed by either charging or discharging processes during this phase. Pulse charging offers the advantage that this charging method extends the service life of electrical energy storage devices compared to constant current methods.

In a preferred embodiment, a special battery charging function is used depending on battery specifications. These battery specifications can be calculated from the input variables of the control unit or supplied by a separate battery management system. This prevents the selection of charging and discharging rates that, while appearing to be energetically favorable, are currently suboptimal for the battery, for example, due to load limits (thermal and/or electrical) and/or that unduly accelerate battery aging. By taking the battery specifications into account with regard to the battery charging function, efficiency is improved and the service life of the battery is extended.

The target SOC is adjusted during operation. The flexible configuration of the target state of charge in relation to driving conditions allows, for example, regenerative operation to occur more frequently, which in turn reduces fuel consumption.

Furthermore, the output variables of the control unit are subjected to a correction, taking into account further system limits and/or special operating conditions, such as the heating of a catalytic converter or a lambda probe, as well as the lambda signal (e.g., in the case of mixture enrichment due to component protection requirements). In this type of optimum value control, in which input variables are monitored to control a process, correction data for catalyst heating can, for example, result in pollutant emissions being reduced earlier in urban operation, for example, derived from driving situations.

The correction information is fed back as input variables. This ensures that the corrections are taken into account in future selections in the form of feedback control. With this type of control, the focus is on maintaining the output value of a process, i.e., ensuring that the specified values lie within a sufficient range of target values.

Preferably, the control unit is designed as an engine control unit of the internal combustion engine. In principle, the charging strategy can be stored via a transmission control unit, a separate hybrid control unit (if available), or even in the control unit of the electric motor. It must only be ensured that the control unit can influence the torque requirements of the internal combustion engine and the electric motor.

• 1 ein schematisches Blockschaltbild eines parallelen Hybridantriebs,
• 2 eine schematische Darstellung der Auswahl aus verschiedenen Lade- und Entladefunktionen und
• 3 eine beispielhafte Darstellung der Arbeitsweise des Verfahrens.

The invention is explained in more detail below using a preferred embodiment. The figures show:

• 1 a schematic block diagram of a parallel hybrid drive,
• 2 a schematic representation of the selection of different charging and discharging functions and
• 3 an example of how the procedure works.

The hybrid drive 1 comprises an internal combustion engine 2, an electric motor 3, an energy storage device 4, which is preferably designed as a battery, a transmission 5, a starting element 6 (for example, a dual clutch or converter), an engine control unit 7, and a transmission control unit 8. The connection of the electric motor 3 to the engine crankshaft can be achieved in various ways, with the connection shown here being via a clutch 9. Alternatively, the electric motor can also be connected directly to the crankshaft of the internal combustion engine or coupled via a belt drive or a transmission. The internal combustion engine 2 is controlled via the engine control unit 7, and the transmission 5 via the transmission control unit 8. Furthermore, the electric motor 3 is assigned a control unit (not shown) and a converter. If necessary, a separate hybrid control unit 10 can also be provided. Control units 7, 8, and 10, as well as the electric motor control unit (not shown), are connected to each other via a bus system 11, for example, a CAN or FlexRay bus. A method is implemented in one of control units 7, 8, and 10 to determine the charging strategy of the hybrid drive. This control unit influences both the internal combustion engine 2 and the electric motor 3.

In the 2 The structure of the charging function according to the invention is shown schematically. In a strategy block 13, a selection is made as to which charging or discharging function is activated based on the input variables 12, including the current difference between the actual SOC and the target SOC, the distance between the actual SOC and (variable, e.g., "soft" and "hard") SOC min/max limits, the current operating points of the combustion engine and electric motor as well as their efficiency maps, battery parameters (e.g., SOH temperature), gear information, vehicle speed, the state of the separating clutch, and other operating parameters of the engine and transmission.

It is preferred to provide the multitude of possible input variables with importance factors and thus to make the selection in a kind of fuzzy logic.

The basic calculation of battery charge (charging I, II, III ... up to n - equivalent discharging) through load point shifting can be performed, for example, using a characteristic map that receives, among other input variables, the speed of the internal combustion engine and the electric motor, as well as the combustion engine torque before the load point shift. Furthermore, if the actual SOC and the target SOC are equal or deviate only slightly, no charging can take place (zero charge) or pulse charging can be used. Furthermore, direct charging and discharging requirements of the battery, which arise due to load limits (thermal/electrical) or aging processes, must be taken into account.

The output values are preferably power or torque values of the electric machine.

When the selection in the strategy block changes, switching between individual charging functions takes place using the switching block 14 or interpolation is carried out based on the current output values for the generator power (or torque) or electromotive power (or torque) of the electric machine 3.

In the final block 15, the output values are corrected. This may be necessary, for example, if further system limits are reached, the combustion engine is in special operating states (e.g., catalyst heating, component protection/enrichment, driving behavior), or the default values from the battery management system need to be subsequently limited or modified. The correction information also flows back into strategy block 13.

In 3 The procedure is illustrated as an example. In addition to the curves of the target and actual SOC, one (of several possible) upper and lower SOC limits are entered. The operating strategy should initially focus on fuel consumption savings.

After an electric drive has taken place in the first time period up to time 1 – here with the aim of replacing an efficient operating range of the combustion engine with electric drive (with the combustion engine switched off) – this drive is interrupted, i.e., the combustion engine is restarted (for example, due to higher torque requirements or because a (first) lower limit SOC value for electric driving has been reached). The electric drive has caused the deviation shown between the target SOC and the actual SOC, resulting in a corresponding recharging requirement. Since recuperation does not occur immediately afterwards, the energy storage device must be recharged by shifting the load point of the combustion engine.

However, it is generally "speculated" that the energy storage device can be recharged or partially charged through recuperation, i.e., without additional fuel consumption. In this case, an average charging power (Charging II) is initially selected, which results from a compromise between the achievable charging efficiency and charging strength depending on the operating point (combustion engine + electric motor), and can vary accordingly. During the journey, as shown here, the specified target SOC can change simultaneously. In this case, the target SOC is lowered to ensure the greatest possible lead (distance from an upper SOC limit) for the amount of recuperable energy.

When the SOC value is reached at time 2, the charging power is switched to the charging power according to Charging I, since there is only a slight deviation from the target SOC. Here, too, high priority is given to an efficient charging process, while at the same time the charging power is set as low as possible, with the goal of discharging the remaining charge through a recuperation process (if necessary). At time 3, recuperation takes place through braking. This recharges the battery, resulting in an actual SOC value that is even higher than the target SOC value.

Immediately following this, a boost process begins at time 4, which very quickly empties the energy storage device to a low SOC value. The reproducibility of driving behavior takes priority over consumption. In order to ensure sufficient energy is available in the storage device for a subsequent boost process, the Charge III function is activated. This function aims to recharge the energy storage device as quickly as possible, with high priority in terms of charging power and at the expense of charging with optimal efficiency. At time 6, the Charge II charging function is activated because the difference between the target SOC and the actual SOC has already decreased considerably, thus offering the opportunity to operate more consumption-oriented, i.e., with higher priority in terms of charging efficiency. In this case, the target SOC still increases in parallel to create a corresponding reserve in the energy storage device for subsequent boost processes (at the expense of the energy storage buffer for recuperation). Alternatively, Charge I can also be activated here before the target SOC is reached.

list of reference symbols

1

Hybrid drive

2

internal combustion engine

3

electric machine

4

Energy storage

5

transmission

6

Starting element

7

Engine control unit

8

Transmission control unit

9

coupling

10

Hybrid control unit

11

bus system

12

Input variables

13

Strategy block

14

Switching block

15

block

Claims (7)

Method for the charging strategy of a hybrid drive (1) by means of an internal combustion engine (2), an electric machine (3), a battery and at least a control unit, - wherein various charging and discharging functions for the battery are stored in the control unit, - wherein, depending on various input variables, the control unit selects one of the various charging or discharging functions, which is then set by a load point shift on the internal combustion engine (2) and the electric machine (3), - wherein the input variables are at least a current difference between an actual state of charge and a target state of charge, a distance from the actual state of charge to min/max limits of the state of charge and current operating points of the internal combustion engine (2) and the electric machine (3) as well as their efficiency maps, - wherein, in addition to the charging and discharging functions, a 0-charge function and a pulse charge function are also stored in the control unit, - wherein the target state of charge is adjusted during operation of the hybrid drive (1), - wherein output variables of the control unit are subjected to a correction, wherein further system limits and/or special operating states are taken into account and wherein correction information is fed back as input variables. Procedure according to Claim 1 , characterized in that further battery parameters, gear information, vehicle speed, state of a separating clutch and/or further operating parameters of the internal combustion engine (2) and/or a transmission (5) are taken into account as further input variables. Method according to one of the preceding claims, characterized in that a special battery charging function is used depending on battery specifications. Control unit for implementing a method for the charging strategy of a hybrid drive (1), - wherein the control unit selects one of various charging or discharging functions depending on various input variables, which is then adjusted by shifting the load point on an internal combustion engine (2) and an electric motor (3), - wherein the input variables are at least a current difference between an actual state of charge and a target state of charge, a distance from the actual state of charge to the min/max limits of the state of charge, and current operating points of the internal combustion engine (2) and the electric motor (3), as well as their efficiency maps, - wherein, in addition to the charging and discharging functions, a zero-charge function and a pulse-charge function are also stored in the control unit, - wherein the control unit adjusts the target state of charge during operation of the hybrid drive (1), - wherein output variables of the control unit are subjected to a correction, taking into account further system limits and/or special operating conditions, and wherein correction information is fed back as input variables. Control unit after Claim 4 , characterized in that further battery parameters, gear information, vehicle speed, state of a separating clutch and/or further operating parameters of the internal combustion engine (2) and/or a transmission (5) are taken into account as further input variables. Control unit according to one of the Claims 4 or 5 , characterized in that a battery charging function is additionally stored, which is selected depending on battery specifications. Control unit according to one of the Claims 4 until 6 , characterized in that the control unit is designed as an engine control unit (7) of the internal combustion engine (2), as a transmission control unit (8), as a hybrid control unit (10) or as a control unit of the electric machine (3).

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